1. Field of the Invention
The present invention relates to an audio muting circuit and an audio muting method. In particular, the invention relates to an audio muting circuit and an audio muting method for muting an audio signal.
2. Description of the Background Art
An audio muting circuit provided in such audio-related equipment as a DVD (Digital Video Disc) player performs mute operation for silencing an audio output. A conventional audio muting circuit generates a short sharp “pop” noise (pop noise) at the start of muting.
FIG. 3 is a time chart showing waveforms of a mute signal of a conventional audio muting circuit and an audio signal appearing at an output terminal of the audio muting circuit. Referring to FIG. 3, in a silent state, the audio signal is at a potential of 0 V. At time t11, in response to fall of the mute signal from “H” level which is inactivation level to “L” level which is activation level, the potential of the audio signal becomes slightly higher than 0 V. Because of this, “pop” noise is generated at the start of muting.
Japanese Patent Laying-Open No. 2000-341047 for example discloses a technique that is applied to an audio power amplifier for which audio environment is important and that is used for preventing pop noise generated when power is supplied and when muting is cancelled, without dependence on a time constant capacitor element which is difficult to incorporate into a semiconductor integrated circuit. With this technique, an equivalent impedance Rx which can be varied in stepwise manner is used to control a transmission gain of an audio signal to accomplish stepwise and gradual cancellation of muting when power is supplied.
Further, Japanese Registered Utility Model No. 3073853 discloses an audio muting apparatus that uses a D/A converter to generate a signal with a gentle gradient that is necessary for muting, so that a low-pass filter can be dispensed with. With this apparatus, muting is accomplished by gradually changing a converted output voltage of the D/A converter from an initial voltage toward a predetermined voltage, and the muting is cancelled by gradually changing the converted output voltage from the predetermined voltage toward the initial voltage.
Furthermore, Japanese Patent Laying-Open No. 2003-289222 discloses an inverting amplifier with muting capability that exhibits superior muting performance while preventing an increase in power consumption, in spite of its simple circuit configuration.
As seen from the above, the conventional audio muting circuits have the problem that “pop” noise is generated at the start of muting.